Boxman etal
chroma key circuit

ABSTRACT

A CIRCUIT FOR GENERATING AN OUTPUT PULSE UPON THE OCCURRENCE OF A SELECTED COLOR IN COLOR TELEVISION SIGNALS FEATURES A MATRIX TO FORM TWO CHROMINANCE DIFFERENCE SIGNALS. FOUR VARIABLE GAIN AMPLIIFERS MULTIPLY THE DIFFERENCE SIGNALS BY THE SINCE AND COSINE OF A CONTROL VOLTAGE WHICH IS A COURCE COLOR COLOUR. TWO CIRCUITS EACH COMPRISING A SUMMER, INVERTER, AND CLAMP CIRCUIT EACH ADD THE OUTPUTS OF TWO OF THE MULTIPLIED SIGNALS. AN AND GATE COMBINES THE OUTPUT OF THE SUMMER CIRCUITS AND ALSO IS COUPLED TO A VARIABLE BIAS SUPPLY, WHICH IS A FINE COLOR CONTROL. A DELAY LINE, A SECOND AND GATE, AND A CLIPPER AND ZERO REFERENCE THE SHAPE THE OUTPUT PULSE.

May 28, 1974 p BOXMAN E TAL Re. 28,021

CHROMA KEY CIRCUIT Original Filed March 12, 1971 4 Sheets-Sheet 1 LLIt() u) om Q CLS m 2 D. O l o Q P# O 2 p OIL Y!" L'JO U xu, o D i 21.1.12E N 23m & OLr o z' n: r n L o E L89] O 05 O)- O L O Ixm Q 2 1 2 uJ O D Dn. DI (D CIJ 21 l: o C 5 B O INVENTORS PETER BOXMAN Y FREDERIK J. VANROESSEL 4 Shsnts`-Shaot 2 Original Filed March l2, 1971 May 28, 1974 p.BoxMAN ETAL CHROMA KEY CIRCUIT Original Filed March 12, 1971 FIG.3

4 Shoets-$heo t 5 FIG. 3b

1.25 FIG. 3c

FIG. 4b

BYFREDERIK INVENTORS PETER BOX MAN J-VAN ROEBSEL M AGE May 28, 1974 p,BOXMAN ETAL Re.

OHRDHA KEY CIRCUIT Original Filed March 12, 1971 4 Sheets-Sheet 4 FROM34 l AND To DELAY FROM 35 f LINE 37 I 3e ro/ 71K?"- BIA S\74 FIG. 5

FROM 34 To DELAY UNE 37 FROM 35 FIG. 6

INVENTORS PETE R BOX MAN FREDE RIK J. VAN ROESSEL United States PatentOliice Reg,... M., 2 1.,..

28,021 CHROMA KEY CIRCUIT Peter Boxman, Clifton, and Frederik Johannesvan Roessel, Mahwah, NJ., assignors to Philips Broadcast EquipmentCorporation, Montvale, NJ.

Original No. 3,678,182, dated July 18, 1972, Ser. No. 123,690, Mar. 12,1971. Application for reissue Sept. 13, 1972, Ser. No. 288,567

Int. Cl. H0411 9/02 U.S. Cl. 178-5.4 R 12 Claims Matter enclosed inheavy brackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT F THE DISCLOSURE A circuit for generating an output pulse uponthe occurrence of a selected color in color television signals featuresa matrix to form two chrominance dilierence signals. Four variable gainamplifiers multiply the difference signals by the sine and cosine of acontrol voltage which is a course color control. Two circuits eachcomprising a summer, inverter, and clamp circuit each add the outputs oftwo of the multiplied signals. An AND gate combines the output of thesummer circuits and also is coupled to a variable bias supply, which isa line color control. A delay line, a second AND gate, and a clipper andzero reference then shape the output pulse.

BACKGROUND This invention relates to a circuit for cutting oli the videosignal of one camera and substituting that of another camera during theoccurrence of a selected color from said lirst camera.

Typical prior art devices utilize simple variable resistor matrices toselect the trigger color. However, such systems have `very poor colorselectivity, and are cumbersome to operate by remote control becausethere are three controls.

1t is therefore an object of the present invention to have chroma keycircuit with a high degree of color selectivity.

It is another object to have such a circuit suitable for remote control.

It is still another object to have such a circuit controlled by a singleknob control.

In brief, these and other objects are achieved by means of a pluralityof variable gain amplifiers which multiply the color signals by the sineand cosine function of a single control voltage, which selects thetrigger color. The multiplied signals are then combined in summers, andthen are applied to an AND gate. A variable bias supply can be connectedto the AND gate to act as a ne color control.

Other objects, and advantages will become apparent from the descriptionwhen, taken in conjunction with the drawings in which:

FIG. 1 is a block diagram of the overall system;

FIG. 2 is a block diagram of the chroma key circuit;

FIG. 3 is a schematic diagram of a block in FIG. 2;

FIGS. 3 a, b, c are graphs of voltage transfer functions of FIG. 3;

FIG. 4 is a schematic diagram of another block in FIG. 2;

FIGS. 4 a and b are graphs of voltage transfer functions of FIG. 4; and

FIGS. 5 and 6 are alternate embodiments of a block in FIG. 2.

FIG. `l shows the basic overall system of the invention. A tirst camerais viewing a scene consisting of, for example, a person l1 against abackground 12, which for example, may be blue in color. A second camera13 is viewing a background scene 14, such as a street scene. alandscape, or other forms of desired background. It is desired to addthe gure 11 into the background scene 14 in the tinal video outputsignal. To accomplish this in accordance with the invention, the systemcomprises an encoder 15, which takes the red, green, and the blue colorcomponents coming from the camera 10 and encodes them into a singlecomposite signal. Assume that the camera 1I] is scanning along aparticular scanning line 16, and at a particular instant the videosignal coming from camera 10 corresponds to the head of gure 11. Thenthe video signal from the encoder 1S will pass through the switch 17,which is in the position shown by the solid line, and along a cable 18to the adder 19. The lower half of switch 17, which is also in theposition indicated by the solid lines, is coupled to a source ofpositive pedestal voltage, which is added to the output of the encoder15 to prevent the output signal from going negative thereby to preventdistortion. At the location of camera 13, switch 20 is in the positionshown by solid lines, and therefore, a source of negative pedestalvoltage is applied to the adder 19 so that it cancels the positivepedestal voltage applied at switch 17. Therefore, the output of adder 19is a video signal representing the ligure 11, and this signal is passedon to other circuits (not shown) within the television studio. Since, inthe assumed example, the chroma key generates a pulse upon theoccurrence of a blue signal, when the camera 10 is scanning the bluebackground 12, which is to the right of figure 1l, the chroma key 90will generate a control pulse to move the switch 17 into the lowerposition as shown by the dotted lines. This chroma key pulse will thengo through the lower portion of switch 17, along the cable 18, and intothe command pulse regenerator 21. Here the key pulse will be regeneratedand will actuate the switch 20 to go into the lower dotted position. Inthis position the video signal from camera 13 will go through switch 20and on to the other portions of the television studio. Therefore, itwill be seen that the video signal from the ligure 11 will besuperimposed upon the background scene 14 in the output composite videosignal. In general, it is desired to have the chroma key 90 adjustable,so that it will generate a keying pulse upon the occurrence of aselected color. This enables its use with any color for the background12.

FIG. 2 shows in more detail the adjustable chroma key pulse generator90. The input red, green, and blue color component signals from thecamera 10 are applied to a matrix 22 which forms two output signals,(B-M), and -(RM). By M is meant a luminance signal, which can becomposed of equal proportions of the R,G, and B signals, or otherproportions which may give better results. As is well known in the art,the color diierence signals contain the same information as the originalcolor signals albeit in dilierent form. The first of these signals,(B-M), is applied to the inputs of two variable gain amplifiers 23 and24; and the second of these signals, -(R-M), is applied to the inputs ofvariable gain amplifiers 25 and 26. Amplifiers 23, 24, 25 and 26 haveamplification gain control terminals 27, 28, 29, and 30 respectively Thegain of these amplifiers linearly varies from -2 to +2 over an inputgain control voltage range of 0 to 2 volts. A manually adjustable inputcontrol voltage labelled X varies from 0 to S volts and is applied to DCcontrol voltage generators 31 and 32. Control voltage generator 31generates a triangular approximation to the -sine of the input voltage,while control voltage 32 generates a triangular approximation to thecosine of the input 'voltage X. The -sin X voltage from generator 31 isapplied to terminals 30 and 27 of amplifiers 26 and 23 respectively. Thevariable gain amplifiers generate output signals which are equal to thenegative (due to the phase inversion within the amplifiers) of theproduct of the color and gain control signals. Therefore, amplifiers 23and 26 generate the -(B-M) sin X voltage and the -(R-M) sin X voltagerespectively. The -cos X voltage is applied to terminal 28 of amplifier24, which thereby generates the (B-M) cos X voltage. The -cos X voltageis also applied to inverter 33 whose output voltage, cos X, is in turnapplied to variable gain amplifier 25, which therefor generates the(R-M) cos X voltage. In other words, the color difference signals comingout of the variable gain amplifiers have an amplitude which iscontrolled from the input voltage X in accordance with either the cosineor the sine of X, as determined by the circuits 31 and 32. As is knownin the art, by varying the amplitudes of sinusoidal and cosinusoidalcolor difference signals, any resultant color can be generated. Thus, byvarying the voltage X the color which will cause the key 9|] to generatea key pulse can be selected. The voltage X serves as a course colorselection for the key 90. The outputs of amplifiers 23 and 25 areapplied to circuit 34 which is a summer, inverter, and a clamp, in theorder recited. The output of circuit 34 is therefore a [(B-M)sinX]-[(R-M)cos X] voltage. Likewise the output signals of amplifiers 24and 26 are applied to a circuit 35 which also is a summer, inverter, anda clamp, and which therefore generates a (B-M) oos X+ (RM) sin Xvoltage. The output signais from circuits 34 and 35 are applied to thenon-inverting signals of AND gate 36. This AND gate 36 can be aFairchild type 711I which generates a low output pulse when bothnon-inverting inputs are low, i.e., it is a negative logic AND gate.Applied to the inverting inputs of the AND gate 36, is a variablenegative bias supply 91 which will determine the trigger level of theAND gate 36. This variable bias supply 91 serves as a fine color adjustfor the chroma key circuit. The output signal from AND gate 36 isapplied to a delay line 37, which corrects for delay in the encoder 15.The output signal of delay line 37 is applied to the non-inverting inputof an AND gate 38 which can be a Fairchild type 710 and which hasapplied to its inverting input a bias voltage from supply 92, which isset to about half of the peak value of the pulse, and thus reshapes thepulse coming from delay line 37. The output signal from AND gate 38 isapplied to the clipper and zero reference 39 which clips the Outputpulse to a half a volt maximum value and sets a zero reference tocorrect for tolerance of the components of the system. Thus in operationthe voltage X would be varied to coursely select the background 12 colorupon which a pulse will appear out of the clipper 39, and the variablebias applied to AND gate 36 would select a fine adjustment of thiscolor.

FIG. 3 shows the details of the cosine generator 32 of FIG. 2. Anoperational amplifier 40, which can be a Fairchild type 741, has aninverting input 41, a non-inverting input 42, and an output terminal 32.A feedback resistor 44 coupled between input 43 and the inverting input41 equals the input resistor 45, and therefore the gain of the inputvoltage X through the inverting input 41 to the output 43 is equal to-1. Oppositely poled series connected diodes 46 and 47 are coupledbetween the non-inverting input 42 and the source of input voltage X. Asource of positive bias potential (not shown) is coupled to the terminal48 and it together with the resistor 49 and 50 bias the junction of thediodes 46 and 47 to a potential of +2.5 volts. Therefore, if the inputpotential is below 2.5 volts it will flow through the diodes 46, 47 andinto the terminal 42. The gain between the non-inverting input 42 andthe output 43 is equal to 2, and therefore if the input voltage X isbelow 2.5 volts, the resulting output voltage at terminal 43 will be thesum of the gain through the in-verting input terminal 41, which has again of -l, and the noninverting input 42, which has a gain of +2. Thisresults in overall system gain of +1. This is shown in FIG. 3a, whichshows the output voltage graphed as a function of the input voltage X.When the input voltage X exceeds 2.5

volts the diode 46 becomes reversed biased, opening input terminal 42from the input voltage, and the gain is therefore 1, as is shown in FIG.3b which is drawn with the same horizontal axis as that of FIG. 3a.Therefore, the input-output transfer function of the entire cosinegenerator is shown in FIG. 3c, which shows a first portion increasingfor the first 0 to 2.5 volts of the input signal and a second decreasingportion for the 2.5 to 5 volts range of the input signal. It will beappreciated that if a L25 volts negative bias is applied to the outputvoltage, the overall triangular waves form would be shifted downward bythis amount. This results in an approximation to the -cos X voltage,which is required by the variable gain amplifier 24.

FIG. 4 shows the details of the sine generator 31 of FIG. 2. It alsocomprises an operational amplifier 51 also a Fairchild type 741 havingan output 52, an inverting input 53, and a non-inverting input 54. Thefeedback resistor and the input resistor 56 are equal and therefore thegain of the input voltage X through the inverting input 53 to the ouput52 is equal to 1. The bias supply 57 is coupled through a resistor 58 tothe oppositely poled series connected diodes 59 and 60 which are coupledbetween the non-inverting input 54 and the source of the input voltageX. The resistor 61 is coupled to the diode 60. Assume that the biassupply 57 with the resistor 58 biases the junction of the diodes 59 and60 to that of +1.25 volts, and also assume that the resistor 61 isgrounded at the end that is indicated to be connected to a bias supply66 of +2.5 volts. The aforesaid components will then act similarly tothe corresponding components of FIG. 3, and will give an input outputtransfer function as shown in the right hand portion of FIG. 4a where Xhas a positive value. This is very similar to that shown in FIG. 3cexcept that the break point occurs at X equals +1.25 volts. The otherdiodes of the bridge network, namely diodes 62 and 63, are alsooppositely poled and have their junction connected to a bias resistor 64which is coupled to a `bias supply 65. Assume that the bias supply 65together with the resistor 64 biases the junction of the diodes 62 and63 to a potential of l.25 volts. Therefore, since diodes 62 and 63 arepoled opposite with respect to diodes 59 and 60 respectively, thesecomponents will give a transfer function as is shown in the left hand ofFIG. 4a with a break point of 1.25 volts. Since X, the control voltage,is only a positive value it is desired to shift the entire transferfunction as shown in FIG. 4a to the right. This is accomplished byadding 2.5 volts to the previously mentioned potential of bias supply65, as well as that to the previously mentioned zero potential of biassupply 66 coupled to the resistor 61. This results in the junction ofdiodes 59 and 60 being at a potential of +2.5 and +1.25=+3.75 volts, andthe junction of diodes 62 and 63 being at a potential of +2.51.25=+l.25volts. This moves all the breakpoints to the right as required, and theresults as is shown in FIG. 4b. It will be seen therefore, that thegenerator 31 generates a triangular approximation to the sin X voltage.

FIG. 5 shows alternative ways of biasing the AND gate 36 so as toachieve a more accurate fine color control. As in FIG. 2, the outputsignals from the summer, inverter, and clamp circuits 34, 35 are appliedto the respective non-inverting inputs of the AND gate 36. However,instead of just having a selectable fixed bias on the inverting inputsof the AND gate 36, there is in addition, a bias proportional to thesignal present at the noncorresponding non-inverting input. This isachieved by having the tapped resistor 70 apply a portion of the signalcoming from circuit 34 to the inverting input of AND gate 36 which isassociated with the inverting input of AND gate 36 to which the signalfrom circuit 35 is applied. Likewise the resistor 71 applies a signalcoming from circuit 35 to the inverting input of the AND gate 36 whichis associated with the non-inverting input of said AND gate to which isapplied the signal from circuit 34.

The proportion of these signals which are applied to the invertinginputs are normally equal and would be fixed set initially. To apply aselectable fixed bias to the inverting inputs, a transistor 72 iscoupled as an emitter follower with the resistor 73 coupled to the biassupply 74. The use of an emitter follower circuit is desirable so that alow AC impedance is presented to the lower ends of the resistors, 70,71. A potentiometer 75 applies selectable bias to the base of thetransistor 74, thereby adjusting the amount of potential at the lowerends of the resistors, 70, 7l. In operation the voltage dividing ratioof the resistors, 70, 7l will effect a fine selection of the color uponwhich the overall chroma key will trigger, and the adjustment of theresistor 75 will control the saturation of the color upon which theoverall key will trigger. It will therefore be seen, that extremely finecontrol of the overall circuit is obtained.

FIG. 6 shows a third way of achieving more color selectivity. The signalfrom the circuit 34 is applied a noninverting input and anon-corresponding inverting input of the AND gate 36. A variable biassupply 76 has a positive voltage output coupled to the remaininginverting input and an equal but negative voltage output coupled to theremaining non-inverting input. By adjusting the voltages from biassupply 76, the triggering color is selected. The signal from the circuit35 is applied to the noninverting input of an AND gate 78. A variablebias supply 79 supplies a negative bias to the inverting input of saidAND gate 78, and the output of said AND gate 78 is coupled through adiode 80 to an inhibit terminal 81 on the AND gate 36. By adjusting thevariable bias supply 79, the saturation of the color on which the chromakey is to trigger can be selected.

It is to be understood that many other embodiments are possible withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. A circuit for generating an output pulse upon the occurrence of aselected color in a plurality of color component signals, comprisingmeans for receiving a control voltage, means for generating a. voltagehaving a value substantially equal to the sine of said control voltage,means for generating a voltage having a value substantially equal to thecosine of said control voltage; first means for multiplying a first ofsaid component signals by said sine voltage, second means formultiplying said first component signal by said cosine voltage, thirdmeans for multiplying a second of said component signals by the negativeof said cosine voltage; fourth means for multiplying said secondcomponent signal by said sine voltage; first means for adding themultiplied signals from said first and third multiplier means, secondmeans for adding the multiplied signals from said second and fourthmultiplier means, and means for combining the outputs of both of saidadding means, thereby to produce an output pulse upon the occurrence ofa selected color in said component signals.

2. A circuit as claimed in claim 1 further comprising a matrix having aninput for receiving red, blue, and green color signals and an outputmeans for applying luminance color difference signals to said respectivemultiplier means, said luminance color difference signals being saidcolor component signals.

3. A circuit as claimed in claim 1 wherein said each of said generatingmeans comprises an amplifier having inverting and non-inverting inputs,and an output; a feedback resistor coupled between said inverting inputand said output; an input resistor coupled to receive said controlvoltage and to said inverting input; a first pair of oppositely poledseries coupled diodes having an end coupled to said non-inverting inputand a remaining end coupled to receive said control voltage; and firstmeans for biasing the junction of said first pair of diodes.

4. A circuit as claimed in claim 3 wherein said cosine generating meanssaid biasing means biases the junction to a potential approximatelyequal to one-half of said control voltage maximum value.

5. A circuit as claimed in claim 3 wherein said sine generating meanssaid first biasing means biases said junction to a potentialapproximately equal to three-quarters of said control voltage maximumvalue, and further comprising a second pair oppositely poled seriescoupled diodes, said pair having its ends coupled to said rst diode pairends respectively; and second means for biasing the junction of saidsecond diode pair to a potential approximately equal to one-quarter ofsaid control voltage maximum value.

6. A circuit as claimed in claim 1 wherein said combining meanscomprises an AND gate having two non-inverting inputs coupled to saidadding means respectively, two inverting inputs corresponding to saidnon-inverting inputs respectively, and an output for providing saidoutput pulse; and means for biasing said inverting inputs.

7. A circuit as claimed in claim 6 wherein said biasing means comprisesa bias supply connected to said inverting inputs.

8. A circuit as claimed in claim 6 wherein said biasing means comprisesa pair of tapped resistors each having one end coupled to saidnon-inverting inputs respectively, said taps being coupled to thenon-corresponding inverting inputs respectively; and a bias supplycoupled to the remaining ends of said resistors.

9. A circuit as claimed in claim 8 further comprising a transistoremitter follower coupled between said bias supply and said resistors.

10. A circuit as claimed in claim 1 further comprising a delay linecoupled to said combining means; an AND gate having a non-invertinginput coupled to said delay line, an inverting input, and an output; abias supply coupled to said inverting input; and a clipping and zeroreference means coupled to said AND gate output.

1l. A circuit as claimed in claim 1 wherein said combining meanscomprises a first AND gate having two inverting inputs, twocorresponding non-inverting inputs, an inhibit input, and an output forproviding said output pulse; an inverting input and thenon-corresponding noninverting input being coupled to one of said addingmeans; means for variably biasing said remaining inverting andnon-inverting inputs; a second AND gate having an inverting and anon-inverting input, and an output coupled to said inhibit input; saidsecond AND gate non-inverting input being coupled to the remainingadding means; means for variably biasing said second AND gate invertinginput.

I2. A circuit for generating an output signal upon the occurrence of aselected color in at least two color component signals, said circuitcomprising means for receiving a control voltage and for generating apair of voltages having values substantially equal `to the sine andcosine 0f said control voltage respectively, four means for multiplying,each of said component signals by one of said sine and cosine voltagesrespectively, one of said multiplied signals having a sign opposite fromthe remaining multiplied signals, a pair of means for adding saidmultiplied signals in combination of two signals, and means forcombining both of said added signals and for producing a pulse upon theoccurrence of a selected color signal.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,723,307 11/1955 Baracket et al. 178-DIG. 63,595,987 7/1971 Vlahos 178-5.2 R 3,619,495 11/1971 ITO et al. 178--6.8

RICHARD MURRAY, Primary Examiner U.S. Cl. X.R. 178--DIG. 6, 5.2 R

